'\"
'\" Copyright (c) 1989-1993 The Regents of the University of California.
'\" Copyright (c) 1994-1996 Sun Microsystems, Inc.
'\" Copyright (c) 2002 Kevin B. Kenny <kennykb@acm.org>.  All rights reserved.
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
'\"
.TH Tcl_CreateTrace 3 "" Tcl "Tcl Library Procedures"
.so man.macros
.BS
.SH NAME
Tcl_CreateTrace, Tcl_CreateObjTrace, Tcl_DeleteTrace \- arrange for command execution to be traced
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
Tcl_Trace
\fBTcl_CreateTrace\fR(\fIinterp, level, proc, clientData\fR)
.sp
Tcl_Trace
\fBTcl_CreateObjTrace\fR(\fIinterp, level, flags, objProc, clientData, deleteProc\fR)
.sp
\fBTcl_DeleteTrace\fR(\fIinterp, trace\fR)
.SH ARGUMENTS
.AS Tcl_CmdObjTraceDeleteProc *deleteProc
.AP Tcl_Interp *interp in
Interpreter containing command to be traced or untraced.
.AP int level in
Only commands at or below this nesting level will be traced unless
0 is specified.  1 means
top-level commands only, 2 means top-level commands or those that are
invoked as immediate consequences of executing top-level commands
(procedure bodies, bracketed commands, etc.) and so on.
A value of 0 means that commands at any level are traced.
.AP int flags in
Flags governing the trace execution.  See below for details.
.AP Tcl_CmdObjTraceProc *objProc in
Procedure to call for each command that is executed.  See below for
details of the calling sequence.
.AP Tcl_CmdTraceProc *proc in
Procedure to call for each command that is executed.  See below for
details on the calling sequence.
.AP ClientData clientData in
Arbitrary one-word value to pass to \fIobjProc\fR or \fIproc\fR.
.AP Tcl_CmdObjTraceDeleteProc *deleteProc in
Procedure to call when the trace is deleted.  See below for details of
the calling sequence.  A NULL pointer is permissible and results in no
callback when the trace is deleted.
.AP Tcl_Trace trace in
Token for trace to be removed (return value from previous call
to \fBTcl_CreateTrace\fR).
.BE
.SH DESCRIPTION
.PP
\fBTcl_CreateObjTrace\fR arranges for command tracing.  After it is
called, \fIobjProc\fR will be invoked before the Tcl interpreter calls
any command procedure when evaluating commands in \fIinterp\fR.
The return value from \fBTcl_CreateObjTrace\fR is a token for the trace,
which may be passed to \fBTcl_DeleteTrace\fR to remove the trace.
There may be many traces in effect simultaneously for the same
interpreter.
.PP
\fIobjProc\fR should have arguments and result that match the type,
\fBTcl_CmdObjTraceProc\fR:
.PP
.CS
typedef int \fBTcl_CmdObjTraceProc\fR(
        \fBClientData\fR \fIclientData\fR,
        \fBTcl_Interp\fR* \fIinterp\fR,
        int \fIlevel\fR,
        const char *\fIcommand\fR,
        \fBTcl_Command\fR \fIcommandToken\fR,
        int \fIobjc\fR,
        \fBTcl_Obj\fR *const \fIobjv\fR[]);
.CE
.PP
The \fIclientData\fR and \fIinterp\fR parameters are copies of the
corresponding arguments given to \fBTcl_CreateTrace\fR.
\fIClientData\fR typically points to an application-specific data
structure that describes what to do when \fIobjProc\fR is invoked.  The
\fIlevel\fR parameter gives the nesting level of the command (1 for
top-level commands passed to \fBTcl_Eval\fR by the application, 2 for
the next-level commands passed to \fBTcl_Eval\fR as part of parsing or
interpreting level-1 commands, and so on). The \fIcommand\fR parameter
points to a string containing the text of the command, before any
argument substitution.  The \fIcommandToken\fR parameter is a Tcl
command token that identifies the command to be invoked.  The token
may be passed to \fBTcl_GetCommandName\fR,
\fBTcl_GetCommandInfoFromToken\fR, or \fBTcl_SetCommandInfoFromToken\fR to
manipulate the definition of the command. The \fIobjc\fR and \fIobjv\fR
parameters designate the final parameter count and parameter vector
that will be passed to the command, and have had all substitutions
performed.
.PP
The \fIobjProc\fR callback is expected to return a standard Tcl status
return code.  If this code is \fBTCL_OK\fR (the normal case), then
the Tcl interpreter will invoke the command.  Any other return code
is treated as if the command returned that status, and the command is
\fInot\fR invoked.
.PP
The \fIobjProc\fR callback must not modify \fIobjv\fR in any way.
.PP
Tracing will only occur for commands at nesting level less than
or equal to the \fIlevel\fR parameter (i.e. the \fIlevel\fR
parameter to \fIobjProc\fR will always be less than or equal to the
\fIlevel\fR parameter to \fBTcl_CreateTrace\fR).
.PP
Tracing has a significant effect on runtime performance because it
causes the bytecode compiler to refrain from generating in-line code
for Tcl commands such as \fBif\fR and \fBwhile\fR in order that they
may be traced.  If traces for the built-in commands are not required,
the \fIflags\fR parameter may be set to the constant value
\fBTCL_ALLOW_INLINE_COMPILATION\fR.  In this case, traces on built-in
commands may or may not result in trace callbacks, depending on the
state of the interpreter, but run-time performance will be improved
significantly.  (This functionality is desirable, for example, when
using \fBTcl_CreateObjTrace\fR to implement an execution time
profiler.)
.PP
Calls to \fIobjProc\fR will be made by the Tcl parser immediately before
it calls the command procedure for the command (\fIcmdProc\fR).  This
occurs after argument parsing and substitution, so tracing for
substituted commands occurs before tracing of the commands
containing the substitutions.  If there is a syntax error in a
command, or if there is no command procedure associated with a
command name, then no tracing will occur for that command.  If a
string passed to Tcl_Eval contains multiple commands (bracketed, or
on different lines) then multiple calls to \fIobjProc\fR will occur,
one for each command.
.PP
\fBTcl_DeleteTrace\fR removes a trace, so that no future calls will be
made to the procedure associated with the trace.  After \fBTcl_DeleteTrace\fR
returns, the caller should never again use the \fItrace\fR token.
.PP
When \fBTcl_DeleteTrace\fR is called, the interpreter invokes the
\fIdeleteProc\fR that was passed as a parameter to
\fBTcl_CreateObjTrace\fR.  The \fIdeleteProc\fR must match the type,
\fBTcl_CmdObjTraceDeleteProc\fR:
.PP
.CS
typedef void \fBTcl_CmdObjTraceDeleteProc\fR(
        \fBClientData\fR \fIclientData\fR);
.CE
.PP
The \fIclientData\fR parameter will be the same as the
\fIclientData\fR parameter that was originally passed to
\fBTcl_CreateObjTrace\fR.
.PP
\fBTcl_CreateTrace\fR is an alternative interface for command tracing,
\fInot recommended for new applications\fR.  It is provided for backward
compatibility with code that was developed for older versions of the
Tcl interpreter.  It is similar to \fBTcl_CreateObjTrace\fR, except
that its \fIproc\fR parameter should have arguments and result that
match the type \fBTcl_CmdTraceProc\fR:
.PP
.CS
typedef void \fBTcl_CmdTraceProc\fR(
        ClientData \fIclientData\fR,
        Tcl_Interp *\fIinterp\fR,
        int \fIlevel\fR,
        char *\fIcommand\fR,
        Tcl_CmdProc *\fIcmdProc\fR,
        ClientData \fIcmdClientData\fR,
        int \fIargc\fR,
        const char *\fIargv\fR[]);
.CE
.PP
The parameters to the \fIproc\fR callback are similar to those of the
\fIobjProc\fR callback above. The \fIcommandToken\fR is
replaced with \fIcmdProc\fR, a pointer to the (string-based) command
procedure that will be invoked; and \fIcmdClientData\fR, the client
data that will be passed to the procedure.  The \fIobjc\fR parameter
is replaced with an \fIargv\fR parameter, that gives the arguments to
the command as character strings.
\fIProc\fR must not modify the \fIcommand\fR or \fIargv\fR strings.
.PP
If a trace created with \fBTcl_CreateTrace\fR is in effect, inline
compilation of Tcl commands such as \fBif\fR and \fBwhile\fR is always
disabled.  There is no notification when a trace created with
\fBTcl_CreateTrace\fR is deleted.
There is no way to be notified when the trace created by
\fBTcl_CreateTrace\fR is deleted.  There is no way for the \fIproc\fR
associated with a call to \fBTcl_CreateTrace\fR to abort execution of
\fIcommand\fR.
.SH KEYWORDS
command, create, delete, interpreter, trace
